Soft toric contact lens

ABSTRACT

A soft toric contact lens having spherocylindrical power for correcting astigmatism in humans and utilizing a prism ballast as a means of maintaining correct axial orientation. The degree of prism is greater than about 0.5 diopter, and the peripheral outer surface of the lens is tapered off to a relatively thin edge of constant thickness over a substantial portion of the ballast area.

This application is a continuation-in-part of application Ser. No.306,438 filed Sept. 28, 1981, now U.S. Pat. No. 4,508,436.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to contact lenses, and more particularly, to softcontact lenses of the toric type designed to correct astigmatism inhumans.

2. Description of Prior Art

A toric lens has both spherical and cylinder correction and accordinglymust maintain a fixed and predetermined orientation during use. A normalspherical lens tends to rotate in the eye as it floats on tears and issubject to normal blinking and eye movement. The problem of preventingrotation of a toric lens has been addressed in several ways.

A prism ballast has been used in soft contact lenses to provideorientation by gravitational forces, but the greater thickness of thelens over the area of maximum ballast, and particularly the thickness ofthe edge of the lens, has resulted in patient discomfort. To allievatethis problem, the degree of prism has generally been limited to amaximum of from about 0.75 to 1.0 diopters. With lower prism values, thelens is often additionally truncated at the prism base since the ballastalone is insufficient to assure proper orientation. It has also beensuggested to chamfer the edge of the lens over the area of the prismbase to improve patient comfort. These modifications of the lens havenot proved to be entirely satisfactory.

U.S. Pat. No. 4,095,878 discloses a method for assuring axialorientation in which a soft contact lens having a relatively thick edgeis flattened along a portion of its peripheral region in the 12 o'clockand 6 o'clock positions so that the thickness of the edges in theseregions is less than the thickness in the regions of 9 o'clock and 3o'clock. Because the relatively thick edges remaining in the 9 o'clockand 3 o'clock positions may still cause discomfort to the wearer, it issuggested to taper off these regions to a limited degree. The techniqueproposed by this reference has not proved to be entirely satisfactory.

It is accordingly an object of the present invention to provide a softtoric contact lens with a spherocylindrical power to correctastigmatism. It is a further object of this invention to provide a softtoric contact lens using a prism ballast to maintain axial orientation.It is a yet further object of this invention to provide a soft toriccontact lens of improved design for increased user comfort. These andother objects of the invention will be apparent from the ensuingdescription and claims.

SUMMARY

A soft toric contact lens of the present invention is provided witheither an inner or outer toric surface, with the desiredspherocylindrical power and appropriate axis to correct astigmatism, andwith a prism ballast to maintain the correct axial orientation duringuse. The degree of prism is greater than about 0.5 diopter, and mostpreferably is from about 1.0 to 1.5 diopters. The outer surface of thelens has a central optical zone and a peripheral lenticular zone, and asignificant portion of the lenticular zone over the area of the prismbase is tapered off to provide a thin and uniform edge thickness overthe entire tapered region. The resulting lens maintains correctorientation during use and provides improved comfort to the user.

DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of a soft toric contact lens having a cylindricalaxis of 90 degrees.

FIG. 2 is a diagrammatic cross section of the lens of FIG. 1 along line2--2.

FIG. 3 is an edge thickness profile of the lens of FIG. 1.

FIG. 4 is a plan view of a soft toric contact lens having a cylindricalaxis of 45 degrees.

FIG. 5 is a diagrammatic cross section of the lens of FIG. 4 along line5--5.

FIG. 6 is an edge thickness profile of the lens of FIG. 4.

DESCRIPTION OF PREFERRED EMBODIMENT

The soft toric contact lenses of the present invention are preferablycomposed of a hydrophilic polymer and manufactured by lathe cutting adehydrated polymer button to the desired lens shape, thereafterhydrating the lens to obtain the finished, soft lens product. Suitablehydrophilic compositions include various hydroxyethylmethacrylate andN-vinyl pyrrolidone polymers, all of which are well-known in the art.Lathe cutting techniques for soft contact lenses are likewise well-knownin the art. The present invention is accordingly directed to an improvedlens configuration which may be utilized in conjunction withconventional polymer compositions and lens manufacturing techniques.

Turning now to FIG. 1, there is illustrated in plan view soft toriccontact lens 10 which consists of central optical area 11, peripherallenticular area 12, and crescent-shaped tapered-off area 13. Adiagrammatic cross section of the lens of FIG. 1 through line 2--2 isshown in FIG. 2. The illustrated lens has a spherical base curve orconvex inner surface including edge bevel 16 and an outer toric surface.FIG. 2 additionally shows by means of shaded area 14 the originalprismatic base of the lens before cutting bevel 13 whereupon thematerial in shaded area 14 is removed to reduce the edge thickness ofthe lens in that region.

Referring again to FIG. 1, the orientation of the lens is indicated byreference to degrees as illustrated. The lenses illustrated anddescribed herein are characterized by having the prism ballast centeredat 270 degrees as indicated by the symbol Δ. The axis of the cylindricalcorrection is indicated by the reference symbol x. As illustrated inFIG. 1, the cylindrical axis of the lens is 90 degrees.

Referring now to FIG. 3, there is diagrammatic illustration of the edgethickness profile proceeding around the lens of FIG. 1 from A to A'. Asa result of incorporating prismatic ballast into the lens, the thicknessof the edge increases slightly in proceeding from A to B and from A' toB'. As a result of the present invention, however, edge thicknessremains constant from B to B' over the tapered-off area of bevel 13. Theoriginal thickness profile of the lens prior to cutting bevel 13 isindicated by shaded area 15.

In FIG. 4, there is illustrated a soft toric contact lens similar tothat of FIG. 1, but with the cylindrical axis at 45 degrees. Bevel area23 in the lens of FIG. 4 is seen to shift in the direction of thecylindrical axis due to the increase in lens thickness along thecylindrical axis. The extent of the shift of bevel 23 is dependent uponthe power of the cylindrical correction.

FIG. 5 is a diagrammatic cross section of the lens of FIG. 4 along line5--5. In FIG. 5, the material removed from the ballast region of thelens as a result of cutting bevel 23 is illustrated by shaded area 24.

Referring now to FIG. 6, the edge thickness profile of the lens of FIG.4 prior to cutting bevel 23 is indicated by shaded area 25. As a resultof cutting bevel 23, edge thickness is reduced to a constant value overthe span of the bevel from D to D'.

As illustrated in FIGS. 2 and 5, the tapered-off region of the lens issmoothly beveled from the outer surface of the lens along a crescentshaped line of demarcation and without creating any step or shoulderbetween the tapered-off region and the front surface of the lens.

In order to assure maximum comfort to the patient, the tapered-off areaof the lens must extend radially into the lenticular area at least about50 percent, and preferably about 75 percent. If the tapered-off areaextends into the lenticular area less than about 50 percent, the angleof the bevel and/or edge thickness is increased to the discomforture ofthe user. If the tapered area extends more than about 75 percent intothe lenticular area, patient comfort is not appreciably increased, andthe mass of material removed diminishes the effectiveness of the prismballast in stabilizing the orientation of the lens.

While lenses of the present invention are preferably fabricated aspreviously described by lathe cutting dehydrated buttons of ahydrophilic polymer, it will be appreciated that other materials andmanufacturing techniques such as casting or molding of silicone lensescan also be employed to yield comparable soft toric contact lens. Othermodifications or variations in the embodiment described and illustratedherein will be apparent to those skilled in the art and are accordinglywithin the scope of the present invention.

I claim:
 1. A soft toric contact lens of spherocylindrical power havinga concave inner surface, a convex outer surface, a horizontal axis and avertical axis;said outer surface having a central optical zone and aperipheral lenticular zone; said lens including a prism ballast in thevertical axis, the degree of prism being from about 0.5 to 1.0 diopter;the outer surface of said lens having a smoothly beveled tapered-offregion in the lenticular zone in the area of maximum ballast, the edgeof said lens having a substantially uniform thickness in saidtapered-off region.
 2. The soft contact lens of claim 1 wherein saidtapered off region is crescent-shaped.
 3. The soft contact lens of claim1 wherein said concave inner surface is spherical and said convex outersurface is toric.
 4. The soft contact lens of claim 1 wherein theorientation of said spherocylindrical power is in the vertical axis, andsaid tapered-off region is crescent-shaped and symmetrical about saidvertical axis.
 5. The soft contact lens of claim 1 wherein theorientation of said spherocylindrical power is at an angle to thevertical axis, and said tapered-off region is crescent-shaped andnonsymmetrical about said vertical axis.
 6. The soft contact lens ofclaim 1 wherein the degree of prism is about 0.75 diopters.
 7. The softcontact lens of claim 1 wherein said tapered-off region extends radiallyat least about 50 percent into said lenticular zone.
 8. The soft contactlens of claim 1 wherein said tapered-off region extends radially fromabout 50 percent to about 75 percent into said lenticular zone.